The generation of ultrashort light pulses is one of the main lines of research in the field of laser sources. The term “ultrashort pulses” refers to pulses with lengths ranging from hundreds of femtoseconds to picoseconds. These pulses are characterised by a high peak intensity, which can lead to nonlinear effects in different materials. Applications of ultrashort pulses include, for example, medical imaging, Terahertz rays generation and frequency comb generation.
However, it should be noted that there are significant differences between pulses of around 100 fs and pulses under about 30 fs. Because of the Fourier-transform relationship between the temporal and frequency domain descriptions of the laser pulse, the bandwidth of short pulses increases as their time duration decreases. Thus, in the spectral domain, the key feature of ultrashort pulses is that they have large bandwidths.
As the duration decreases, the bandwidth increases inversely. In the near infrared (NIR) 700-3000 nm, a 100 fs pulse has a bandwidth of approximately 8 nm, while a 30 fs pulse has a bandwidth of approximately 40 nm. In the mid-IR from 3000-15000 nm, the required bandwidth is greatly increased over all the pulse durations—this is because the bandwidth-time duration relationship is fundamentally related to the number of optical cycles in the pulse. One period of the electric field, or “cycle” lasts 2.7 fs at 800 nm, and 10.7 fs at 3200 nm. Thus, for a pulse comprising two cycles, the resulting bandwidth is 170 nm Full Width at Half Maximum (FWHM) for 800 nm (NIR) and 700 nm FWHM for 3200 nm (mid-IR). These large bandwidths offer unique advantages for multi-line spectroscopy of gases, as the pulse spectrum simultaneously covers a huge range of absorption lines for different molecules.
Existing high energy sub-100 fs laser pulses typically come from Ti:Sapphire based chirped-pulse amplification (CPA) laser systems. These systems are limited in pulse duration and the wavelength at which they operate due to the gain bandwidth of the Ti:Sapphire amplifier medium, which restricts the output pulse to a spectral range of 600-1100 nm, and a bandwidth of a few tens of nm for high energy systems.
An alternative technique is optical parametric amplification (OPA), which converts energy from a pump pulse to a signal pulse, in a nonlinear crystal, while generating an idler pulse. This has the advantage of amplifying ultrashort pulses with broad bandwidth, and is not limited to a specific wavelength range, but is limited in energy due to the high peak powers, and requires precise synchronisation of the pump and signal lasers.
A combination of the two techniques can be used to produce ultrashort pulses: for example, U.S. Pat. No. 6,873,454 B2 presents a system in which Ti:Sapphire oscillator pulses are first amplified in an OPA and then directed into a Ti:Sapphire amplifier. Further configurations for generating pulses with durations of less than nanoseconds are disclosed in U.S. Pat. No. 7,630,418 B2, also using two different laser sources to feed the system.
US 2009/0244695 A1 presents a different approach to the problem of light source amplification, in this case, using a single oscillator which is divided into two arms, one of which is spectrally broadened while the other one is stretched, amplified in gain storage amplifier, and compressed. The resulting signals from both arms feed an optical parametric amplifier, which is followed by an additional compressor.
Additionally, these systems do not offer the possibility to control the Carrier Envelope Phase (CEP) of the pulses. The CEP is defined as the phase offset between the peak of the amplitude envelope and the peak of the carrier electric field. In the case of few-cycle pulses (that is, pulses formed by only a few periods of the carrier electric field, which means that the pulse has only one or two strong peaks underneath the pulse envelope), CEP is especially relevant as the shape and strength of the electrical field changes as a function of the CEP. Thus, for repeatable operations, a constant CEP for each pulse in a laser pulse train is desired.
Additionally, in order to generate high gain over a broad bandwidth for the amplification of ultrashort pulses, a technique called Optical Parametric Chirped-pulse Amplification (OPCPA) is known. By amplifying temporally stretched pulses (that is, chirped pulses) using optical parametric amplifiers, it is possible to avoid high peak powers in the amplified pulses and hence reach high energies without optical damage. The gain of a OPCPA system for a single pass through a nonlinear crystal is typically much greater than that of a conventional gain storage amplifier. Also, the gain spectrum can be extremely broadband, and can also be centred over a large range of central wavelengths. As a result of the parametric nature of the process, almost no energy is transferred to the amplifier, thus making the system free from thermal lensing and allowing multi-kHz operation with good beam quality. In order to achieve the aforementioned operation over broad gain bandwidths with central wavelengths in different parts of the NIR and mid-IR, restrictive phase-matching conditions must be fulfilled. This include choice of the correct crystal, the use of a high quality pump beam, correct choice of seed and pump pulse durations and in particular accurate temporal synchronisation of the pump and seed beam. This last point in particular has been a strong technical limitation of many of the aforementioned systems.
As a result, there is still a need in the state of the art of a stable ultrashort pulsed light source with a broad bandwidth and which is able to provide an stable CEP.